Common-Mode Voltage in Inverters: Effects and Reduction Methods

Key Takeaways

• The voltage difference between a power source and the neutral point of a load in inverters is called common-mode voltage.

• The effects of common-mode voltage include faults in motors, premature failure of bearings,  unwanted tripping of switchgears, and glitches in the control equipment.

• Incorporating isolation transformers, common-mode chokes, or active and passive filters can reduce the common-mode voltage in inverters. 

Variable frequency drives use inverters to control the amplitude and frequency of the output waveform

Variable frequency drives (VFDs) are power electronic devices used to control the speed and torque of electric motors. VFDs are widely used in many manufacturing industries. They use inverters to control the amplitude and frequency of the output waveform. Inverters convert DC power to AC power of a specified magnitude and frequency. The number of output phases and the number of levels in the output voltage can be determined by the type of inverter used.

Two-level inverters convert DC to AC with two amplitude levels. While a two-level inverter converts DC to AC, it generates total harmonic distortion (THD) and common-mode voltage. The common-mode voltage in inverters is harmful to the motor, especially the bearings. Industries relying on large motors employ common-mode voltage reduction methods to prevent their machines from malfunctioning. In this article, we will explore common-mode voltage, its effects, and some recommended reduction methods. 

Common-Mode Voltage in Inverters

Inverters are commonly used in high-power industrial applications such as interrupted power supplies, AC motor drives, induction heating, etc. Usually, inverters are pulse-width modulated (PWM) for industrial applications. The most commonly used type of PWM is sinusoidal pulse-width modulation (SPWM) due to its rugged control scheme and simplicity. The pulse width is controlled in such a way that the desired output voltage is obtained with reduced harmonic content.

When inverters convert DC to AC power, there is a voltage difference between the power source and the neutral point of the load. This voltage difference in inverters is referred to as common-mode voltage. 

In induction motor drives, the common-mode voltage appears between the start point of the induction motor and the ground. It causes common-mode current to flow through the motor bearings, which is harmful to the machine’s life. Let’s take a look at some of the other effects caused by common-mode mode voltage in inverters in the upcoming section. 

Effects of Common-Mode Voltage in Inverters

Common-mode voltage is considered critical in industries that rely on large motors, especially induction motors. When VFDs utilize inverters, high common-mode voltages are developed. Depending on the pulse-width modulation, the common-mode voltage generated will have high frequency and high amplitude. This common-mode voltage induces shaft voltage in the rotor side of the motor. If the voltage induced in the motor shaft exceeds the breakdown voltage of the lubricant in the bearings, it causes a flow of current in the bearings. This is not good for the motor and ultimately leads to motor failure or malfunctioning. Apart from affecting the motor bearings, current flow in the bearings also causes damage to the output terminals and the electronic components connected to the motor.

Common-mode current due to common-mode voltage in inverters is detrimental to the electrical systems in industries. The effects of common-mode voltage include faults in motors, premature failure of bearings, unwanted tripping of switchgear, glitches in control equipment, etc. When there are common-mode impedance paths in an inverter system, the common-mode voltage allows common-mode current flow at every voltage variation. By producing large common-mode current, common-mode voltages in the inverter worsen electromagnetic interference (EMI). The common-mode voltages in energy storage system-based inverters are capable of causing leakage currents and faulty activation of detection units.

Because common-mode voltages in inverters can cause so much damage, it is necessary to employ common-mode voltage reduction techniques for the extended operation of machinery. 

Common-Mode Voltage Reduction Methods

Multilevel inverters generate lower common-mode voltage compared to two-level or three-level inverters. Therefore, increasing the output voltage levels by using multilevel inverters is one technique that can be employed for reducing the common-mode voltage in an electrical system with inverters. In three-phase inverters, modifying the topology by adding a fourth leg is suitable for reducing the common-mode voltage. Utilizing dual bridge inverters is also a reduction method used for common-mode voltage in conventional inverters.

These reduction techniques are based on hardware circuitry. Similar hardware circuitry-based common-mode reduction methods include incorporating:

• Isolation transformers
• Common-mode chokes or zero sequence impedance
• Active and passive filters 

Many hardware circuit modifications increase the cost and size of the system. The extra money involved in hardware-based common-mode voltage mitigation methods can be overcome by employing modification in the control strategy of inverters. Space vector pulse-width modulation (SVPWM) and modified SPWM are proven to be effective at reducing common-mode voltages in inverters.

Cadence’s suite of design and analysis tools can support you in developing both hardware-based and control strategy-based common-mode voltage reduction methods. Subscribe to our newsletter for the latest updates. If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.